Precipitation

Robert Snyder and Stephan Custer
Geographic Information and Analysis Center and Earth Sciences, Montana State University, Bozeman, MT

Last Modified 7 June 2000; 02 January 2009


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Example Questions the Database Can Help Answer

1.  What is the mean average precipitation expected for the area of interest?
2.  What other climatic information is available for the area of interest?
3.  How much recharge can be expected in the area under review?
        (Note the amount of recharge requires further climatic and soil analysis but requires data on precipitation.)
 

Map 1.  Precipitation

  1. Mean annual precipitation contoured for the period 1961-1990 for part of Gallatin County.
  2. Mean annual precipitation interpolated from contours for the period 1961-1990 for part of Gallatin County.
  3. Prism precipitation data for all of Montana on the web (gif; jpg)
  4. Prism precipitation data for other states on the web.
  5. Natural Resources Conservation Snow Survey Unit Data for the mountains on the web
  6. Climate data by region in the US on the web.
  7. NOAA Climate Data.
  8. NOAA
  9. Bureau of Reclamation Agrimet Data

Glossary

Mean Annual Precipitation is the total annual precipitation calculated from all yearly total precipitation data for the period of interest divided by the number of years in the period of interest.  The values change depending on the period chosen.  The data is usually reported for 30 year periods.  The most recent period available is 1961-1990.  The averages are updated every 10 to 20 years.

Mean Monthly Precipitation is the  total monthly precipitation calculated from all data monthly total precipitation data for the period of interest divided by the number of years in the period of interest.

Contoured mean annual precipitation was drawn by Phil Farnes based upon all available precipitation data for the period 1961-1990.  The contour interval is 2 inches below the 20 inch contour and 10 inches above the 20 inch contour.

Interpolated mean annual precipitation is the precipitation estimated for each grid point based on the known values on the contour lines.    The value at each individual grid point  was calculated from the contoured mean annual precipitation using the program ANUDEM without drainage enforcement.  No digital elevation model was used to enforce the contouring.  Only the precipitation contours were used.

Snow Survey Unit Data.  There are two organizations which routinely collect and summarize data.  The National Oceanic and Atmospheric Administration (NOAA) sites tend to be easily accessible and near where people live.  The Natural Resources Conservation (NRCS) Data tends to be collected at remote high elevation sites.  The NRCS data is used for water supply forecasts in the Western US which are dependent upon knowledge of mountain snow pack and rainfall information.

Precipitation includes both snow and rainfall expressed as water.  Precipitation in the winter is different than snow depth.  Because snow density changes from storm to storm and through the snow-fall season, the snow depth does not reflect directly how much water is in the snow.  Precipitation reflects the water in the snow and in rainfall.
 

About the Data

There are two precipitation maps.  The contour map classifies precipitation both as contour lines (line coverage) and as the mid-point value between the two contours.  The interpolated map shows unique values for each grid point which were calculated from the contour-line data.  The contour map shown is the result of digitization of hand drawn contour maps prepared by Phil Farnes for the period 1961-1990 using all available precipitation data from all known precipitation stations (NOAA and NRCS) in the region.  The work is described in Custer, S.G., Farnes, P., J.P., Wilson, and Snyder, R.D., 1996, A comparison of hand- and spline-drawn precipitation maps for mountainous Montana.  Water Resources Bulletin, v. 32(2), p. 393-405.  The hand-drawn maps are presented because they were drawn by the same person that produced earlier official NRCS precipitation maps.  The map was drawn on the Bozeman 1 x 2 degree sheet.  The lines were digitized, and the value of the mid point between contour lines was assigned to all grid points between contours.  For example, if the contour lines are 18 and 20 inches, the mid point value is 19 inches.  The contour intervals on the map are not uniform.  Below 20 inches, the map was drawn with two inch contours.  Above 20 inches the map was drawn with ten inch contours.   The maps were hand drawn on a 1:250,000 Army Map Service 1x2 degree topographic map (UTM, NAD27), digitized, and reprojected into geographic co-ordinates.  The areas between contours were converted to mid-point values and projected again into UTM NAD83 co-ordinates. The map does not cover all of Gallatin County.  The 1x2 degree sheet does not extend above 46 degrees north latitude nor below 45  degrees north latitude.  No detailed digital data is available north of 46 degrees and south of 45 degrees north latitude.  The precipitation data was clipped so that the data falls between the north-south lines that bound the eastern and western most extent of the county. Because the contours provide no information between the contour lines without interpolation, the data were converted to grid format and interpolated using the program ANUDEM.  Once the data was interpolated, the data was reprojected to UTM NAD 83.  The interpolation map shows unique precipitation values at each grid point rather than the value half way between any two contour lines which is shown on the contour map.  No DEM was used to enforce the interpolation routine to maintain the integrity of the original hand-contoured map to the degree possible.  Only the contour lines were used.  There are some differences between the interpolated results and the contour results which can be seen just west of the LWQD and on the west side of the Bridger mountains.  These differences serve as a caution that the actual precipitation at a given grid point may be different than the interpolated value.    No attribute table is presented because the attribute at each location is simply mean annual precipitation for the period 1961-1990 expressed in inches.

Another widely accepted source of precipitation data is that produced by PRISM.  This information is reported on a map at the web site indicated in item two.  This data reflects computer interpolation of regional data.  PRISM does not include NRCS Snow data and the maps show the entire state so that generalization to a site is difficult.  Several other sites with raw climate data are hyper linked for the interested user.